1. Field of the Invention
The present invention relates to the formation of patterns on photolithography masks used to form patterns on semiconductor wafers. More particularly, the present invention relates to the formation of attenuating phase shift masks used for the fabrication of semiconductor integrated circuits.
2. Description of the Related Art
Semiconductor wafer fabrication involves a series of processes used to create semiconductor devices and integrated circuits (ICs) in and on a semiconductor wafer surface. Fabrication typically involves the basic operations of layering and patterning, together with others such as doping, and heat treatments. Layering is an operation used to add thin layers of material (typically insulator, semi-conductor or conductor) to the surface of the semiconductor wafer. Patterning is an operation that is used to remove specific portions of the top layer or layers on the wafer surface. Patterning is usually accomplished through the use of photolithography (also known as photomasking) to transfer the semiconductor design to the wafer surface. A photomask (also known as a reticle) is used to provide a pattern for transfer to the wafer surface. As the push to maintain Moore's Law is producing smaller and smaller semiconductor circuits, increasing demands are placed upon the process equipment to provide the necessary resolution. To produce the smaller sized devices, the useful lifetime for equipment designed for particular device sizes is often extended by using resolution enhancement techniques (RETs) on the photomasks. For example, significant improvements in resolution and depth of focus have been demonstrated using phase shifting techniques. One of these techniques uses attenuated phase shift masks.
Attenuated Phase Shift Masks include selectively placed patterns of attenuating material, such as, for example, molybdenum silicide (MoSi) on quartz. MoSi typically allows a small percentage of the light to pass through the mask, such as, for example, 6%, in comparison to the approximate 0% allowed by the chrome used in conventional optical lithography masks. However, the thickness of the MoSi is chosen so that the light that does pass through is 180° out of phase with the light that passes through the neighboring clear quartz areas. The light that is transmitted through the MoSi pattern has an intensity that is insufficient to expose the resist. The phase difference causes the intensity of the transmitted light imaged on the wafer photoresist layer to be “darker” than similar features produced using chrome patterns. The result is a sharper intensity profile that allows smaller features to be printed on the wafer. That is, the edges become sharper as a result of the small amount of 180-degree phase shifted light destructively interfering with the light passing through the feature. But chrome still finds use in the formation of alignment marks (i.e., fiducials) on the phase shift masks.
In order to make useful devices the patterns for different lithography steps that belong to a single structure must be aligned to one another. Existing attenuated phase shift masks require a chrome border area around the pattern portions of the mask. The border area contains reticle alignment fiducials and other structures that the stepper needs to perform functions such as bar code recognition and reticle alignment. This is necessitated because the phase shifting films currently used in this scheme (such as MoSi and TiN/SiN) are not sufficiently opaque at the wavelengths that the stepper uses for the above functions. While these materials provide good phase shifting and contrast at the exposure wavelengths (which are in the deep UV regime), they let too much light through at the non-exposure wavelengths, which are in the visible regime. As a result, these peripheral fiducial structures on the reticle cannot be defined in these materials, and require a chrome film and a second (non-critical) chrome patterning step in the mask making process. This second patterning step can cause chrome defects on top of the phase shifting layer, which are hard to clean or remove, thus reducing the overall yield of the process. These steps often result in lower yield and higher manufacturing cost.
Accordingly, it is desirable to provide alternative materials for use in attenuating phase shift masks which have desirable phase shift and transmission characteristics for the formation of patterns yet have sufficient opacity for use in fiducials.